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West's water reservoir managers face big losses from evaporation

As water managers in the western United States increasingly consider new and expanded reservoirs to store more water, they face potentially huge losses from evaporation.

Some are looking at underground storage, covering and shading reservoirs, and adjusting water levels to try to save hundreds of billions of gallons.

Water equivalent to roughly 10 percent of the annual flow in the Colorado River is lost each year to evaporation from just two massive reservoirs, Lake Powell and Lake Mead, according to researchers at the University of Colorado.

That's about 500 billion gallons, perhaps 10 times what metro Denver residents consume in one year, said hydrologist Ben Livneh of CU's Department of Civil, Environmental and Architectural Engineering and the Boulder-based Cooperative Institute for Research in Environmental Sciences.

"By having these big reservoirs, we increase the loss, because reservoirs are big open-water surfaces — optimal surfaces for evaporation relative to the natural landscape," Livneh said.

"This should be a central consideration if new reservoirs are being proposed and built."

Little research has been done on measuring evaporation. Water managers since the 1920s have relied on setting out pans of water next to reservoirs and measuring losses every 24 hours — a method scientist say could be improved.

In October, CU hosted a forum of water managers from around the West with participants from universities, the National Center for Atmospheric Research and government agencies including the U.S. Bureau of Reclamation and the Colorado Water Conservation Board.

A CU-led team is applying for funds to launch a study that would deploy buoys that estimate evaporation based on precise measurements of wind speed, air temperature, water temperature and other factors. The goal is to find out how best to store water while minimizing evaporation.

High-tech buoys have been put in reservoirs in California, Idaho and Nevada. CU geographer Peter Blanken has been studying evaporation from the Great Lakes.

"Water in some reservoirs may evaporate faster than water in others. It's not clear right now where we should be storing it, and where we shouldn't, to minimize evaporation," Livneh said.

"You certainly reduce evaporation losses a lot if you store it underground. ... We want to figure out where to put water so that it is not going to evaporate so much. It may mean moving it around in reservoirs along the Colorado River, or it may mean putting it underground," he said.

"Without reservoirs, we'd be in much worse shape in dry years. We're lucky to have the reservoirs. But reservoirs lead to increased evaporation. The question is, can we reduce it? If we move the water around, could that 10 percent number become 5 percent?"